An electromagnetic relay or contactor in its simplest form consists of a magnetic circuit comprising a fixed core, a moveable armature and one or more air gaps; an electrically energizable actuating coil; one or more sets of contacts; and springs for returning the armature to its unenergized position. When a voltage source of sufficient potential is connected to the actuating coil, current through the coil creates flux in the magnetic circuit. When the flux reaches a value such that the magnetic force on the armature exceeds the spring force and friction forces, the armature will accelerate toward the fixed core. As the air gap between the fixed core and moveable armature decreases, the magnetic circuit reluctance decreases thereby increasing the flux and magnetic force on the armature. Although the spring force opposing armature movement also increases, its increase is substantially linear over the range of motion whereas the flux increase is inversely proportional to the square of the distance. Accordingly, a very strong magnetic force is exerted on the armature at its minimum distance (air gap) from the fixed core.
Although the force on the armature is not itself detrimental, and in some instances may be beneficial in assuring that closed contacts are immune to external vibration, the energy dissipated in the coil is at best inefficient and at worst may overheat the coil and damage it. Recognition of this problem has led to several solutions. Since the actuating current is of necessity initially high in order to generate sufficient flux to move the armature from its rest position, reduction of actuating current is impractical. An alternate solution is to sense armature position using a secondary set of contacts and to reduce coil excitation to a holding current level. Another alternative is to provide a separate holding coil which becomes energized upon contact closure. Both of these alternatives are in current use and both have limitations. For example care must be taken to assure that the magnetic force is maintained sufficiently strong to avoid vibration induced dropouts which can result in oscillation of the contactor. The holding coil approach also may require additional space if a separate coil is formed on the contactor.
It is an object of the present invention to provide an improved contactor energizing system.
It is a further object of the present invention to provide an improved contactor energizing system which regulates coil current at a minimum required value.
It is a still further object of the present invention to provide a contactor energizing system which maintains a constant magnetic force irrespective of contactor air gap.
In accordance with the present invention, an electromagnetic contactor is provided with a fixed air gap in its magnetic circuit and a magnetic flux sensor is placed in the air gap. A controllable voltage source is connected to provide energizing potential to the coil of the contactor. The flux sensor is electrically connected in circuit with the voltage source and is arranged such that an increase in flux in the magnetic circuit above a predetermined level causes a reduction in energizing potential to the coil. Energizing potential increases when flux drops below the predetermined level. Accordingly, the level of magnetic flux generated by the coil is maintained at a substantially constant value selected to provide sufficient force to maintain contact closure without expending excessive energy in the coil.